Many types of machines use fluids such as oil, lubricant or hydraulic fluid. Exemplary machines include vehicle engines, transmissions or manufacturing machines. The fluid quality plays a significant role in the operation and maintenance of the machine and protection of the machine components. Low grade fluids do not adequately protect machine components and can cause damage due to thermal influences and wear. Used fluids that have broken down through use may cause similar damage to the machine components.
Systems have been developed to monitor the fluid quality of these machines. These systems generally include an impedance cell and corresponding instrumentation that measures the impedance of a fluid sample. In one example, an AC signal having a non-zero DC offset is applied to the fluid through an impedance cell and an AC signal is swept across a frequency range from millihertz (mHz) to megahertz (MHz). The measured impedance is compared to reference impedance values to determine the fluid quality.
Inclusion of a DC offset and the AC sweep across a wide frequency range requires a significant amount of time, as well as equipment that is more complicated and more expensive than is desired. Further, impedance data that is measured with one impedance cell is not comparable to impedance data that is measured with an impedance cell of a different design. This is because the geometry of the impedance cell including the size and physical properties of the electrodes influence the impedance data. As a result, if the impedance cell requires replacement, data that was obtained for a fluid with one type of cell cannot be compared to the data generated by another type of impedance cell.